Semiconductor integrated circuit chips and dies are relatively fragile. To protect chips and dies from damage, they are typically confined in a semiconductor package. For example, Ball Grid Array (BGA) packages typically include at least one integrated circuit chip or die mounted and electrically connected to a substrate with conductive trace lines in the substrate connecting the chip or die to electrical contacts on the bottom surface of the substrate. The chip and the substrate are then encapsulated with resin to protect the chip while leaving the electrical contacts on the bottom surface of the substrate exposed.
Substrates used in semiconductor packages can be made from various materials, typically insulators, including ceramic, plastic, and organic, for instance. Electrical trace lines are integrated into or onto the substrate to provide proper power and signals paths for the chip. One way to form electrical traces is illustrated in FIG. 1.
In FIG. 1, an imprinting system 10 is illustrated. The system 10 includes upper and lower platens 12 and 14 that provide pressure for upper and lower rigid microtools 22 and 24, respectively. The microtools 22 and 24 each include several embossings 25 that project from a base of the corresponding microtool. A substrate 30 is covered on two surfaces by impressionable material layers 32 and 34, which can be made of uncured thermal-setting epoxy. As illustrated in FIG. 2, in operation, the microtools 22 and 24 are each pressed by the platens 12 and 14 into the impressionable material layers 32 and 34. Embossings 25 on the microtools 22 and 24 leave impressions in the material layers 32 and 34. The material layers 32 and 34 are then cured. After the microtools 22 and 24 are removed, the impressions remain in the material layers 32 and 34.
The impressions are later filled with an electrically conductive material, such as copper or gold metal, and machined or otherwise processed to provide the electrical traces in the substrate.
After a first set of material layers 32 and 34 is imprinted and the electrical traces created in the layer, another set of material layers of impressionable material (not shown) can be deposited over the first and the cycle can be repeated, resulting in a multi-layer substrate.
Although the platens 12 and 14 are typically compliant or otherwise float to match imperfections of the substrate 30 and the impressionable material layers 32 and 34, the microtools 22 and 24 shown in FIGS. 1 and 2 are relatively rigid. With reference to FIG. 2, when the microtools 22 and 24 are pressed into the impressionable material layers 32 and 34, the embossings 25 do not always imprint to a consistent depth. Because of the imperfections in the substrate 30 and impressionable material layers 32 and 34, known as Total Thickness Variation (TTV), some of the impressions left in the material layers 32 and 34 after the microtools 22 and 24 are removed are relatively shallow or non-existent, while other impressions are relatively deep. This variation in impression depth can cause problems when the impressions are later filled with the conductive material machined to make the electrical traces in the substrate.
During processing of the impressions, the conductive material filling some of the more shallow impressions can be completely or mostly removed, either of which will cause inferior or inoperative electrical connections with the chip or die to be mounted on the substrate.
Another problem with the imprinting process as described is that air or other gasses can be trapped in the material layers 32 and 34, depending on the sequence of events, due to out-gassing of the imprint materials and air pockets formed between the microtools 22, 24 and the material layers 32 and 34.
A soft tooling pressing system is illustrated in FIGS. 3-5. In FIG. 3, a system 40 includes an upper heater 42, lower heater 52, an upper microtool 44 and a lower microtool 54. Differently from the above embodiment, the microtools 44 and 54 are soft tools and have a degree of flexibility and conformity. Soft tool microtools have the ability to conform to variations in the thickness of the impressionable layers. Soft tool microtools can be made from nickel which is relatively flexible. The views of the flexing of the soft microtools 44 and 54 in FIGS. 3-5 are exaggerated for illustration purposes. The substrate 60 can be covered by impressionable materials on both sides.
In a first operation, illustrated in FIG. 4, the microtools 44 and 54 are held in place by a vacuum generated to pull the microtools toward the heaters 42 and 52. As illustrated in FIG. 3, this vacuum action causes a deformation in the soft microtools 44 and 54. Such vacuum action also creates air pockets 70 (shown in FIG. 5) between the microtools 44 and 54 and the substrate 60.
In a next step, imprint pressure is applied to the backsides of the microtools 44 and 54, as illustrated in FIG. 5. As the imprint pressure is applied, the air pockets 70 are partially dissipated, but some of the air from the pockets 70 is forced into the impressionable material on the substrate 60. Other portions are not forced into the material but instead create fluid back pressure that presses on the front sides of the microtools 44 and 54, which can prevent the imprint regions of the microtools from fully pressing into the impressionable material. As described above, this causes connection problems when the impressions are filled and processed into electrical connection lines.
Additionally, because edges of the microtools are clamped in position during the impression period, boundary conditions exist around the microtools 44 and 54 that prevent the tools from ever being able to possibly be perfectly flat and that may adversely affect the impression depth and uniformity.
Embodiments of the invention address these and other disadvantages in the prior art.